1. Field of the Invention
This invention generally relates to a thermal printing apparatus and more particularly, to a thermal printing apparatus for printing multilevel density images, characters and figures which can appropriately regulate the levels of the densities used for representing the gradational images, characters and figures printed by using a thermal head thereof.
2. Description of the Related Art
As a conventional method of printing images, characters or figures by converting input data representing characters or figures to be printed into signals to be applied to a thermal head, the Japanese Patent Application Provisional Publication No. 59-107681 discloses a method for gradational display of characters or figures including steps of generating pulses each having width corresponding to the input data and energizing a thermal head by applying these pulses to the thermal head. In case of this method, the input data are printed by one dot at each line pitch and the printing portion of the thermal head is sequentially moved in a primary scanning direction (see FIG. 7 (a)) of the thermal head.
FIGS. 7(a) and 7(b) are diagrams for illustrating this conventional method for gradational printing of images, characters or Figures. Particularly. FIG. 7 (a) is a diagram for showing a group of resistors for emitting heat arranged in a line composing the linear thermal head. FIG. 7 (b) is a diagram for showing an area, on which dots are printed by these resistors for emitting heat, on the surface of a sheet of recording paper. Further, FIG. 7 (c) is a diagram for showing energizing pulses to be applied to each of the resistors for emitting heat. In these Figures, reference characters R.sub.1, R.sub.2, . . . and R.sub.n indicate resistors for emitting heat of the linear thermal head; 1.sub.TH a pitch (hereunder referred to as a thermal head pitch) or distance between two contiguous dots in the primary scanning direction; 1.sub.THP a pitch (hereunder sometimes referred to as data printing pitch) or distance between two contiguous lines of dots in a secondary scanning direction of relatively moving the thermal head; and t.sub.11 . . . and t.sub.n2 energizing periods of time for energizing the resistors of the thermal head. As shown in these figures, in case of such conventional method, the period of time (hereunder sometimes referred to as energizing period) for energizing each of the resistors R.sub.1, R.sub.2, . . . and R.sub.n is controlled in accordance with gradational printing data of 1 line as to each resistor. Further, the printing of a character is effected by allocating on dot to an area of 1.sub.TH .times.1.sub.THP.
However, in case of using thermal sublimation ink, the above described prior art gradational printing method has encountered the following problems:
(1) The density of a peripheral portion of a printed dot is low. Thus, a portion of low density is generated in each area of 1.sub.TH .times.1.sub.THP with results that sufficient density of the whole area allocated to a dot cannot be obtained.
(2) If the power supplied to the thermal head is increased to obtain further hither density, deformation due to heat is caused on the surface of the recording paper and as a result the density and the luster of an image are decreased.
By the way, the thermal printers are nowadays widely used as the apparatus for producing hard copy to be used for personal computers, word processors, video displays and so on.
The thermal printer records an image by supplying the preferable printing pulses to many heat emitting elements (hereunder sometimes referred to simply as resistors) provided on a thermal head and using thermal dye transfer printing paper or heat sensitive printing paper. In recent years, among such thermal printers, has become widely used an apparatus for printing a gradational image or multilevel density image by supplying printing pulses, of which power correspond to the density levels of an input image, to each heat emitting element.
When recording such a multilevel density image, data represented by using 8 bits or so corresponding to input image signal are employed as input data. Further, the period for energizing the resistors is selected to correspond to the input data. It has been, however, known that if the increment of the period of energizing the resistors to be used to print data having hither density by one level is set as constant, the actual density of a printed image cannot accurately correspond to the input data.
FIG. 21 is a graph showing the relation between the actual density of the printed image and the energizing period for energizing the resistors.
As shown in this figure, the actual density of the printed image is not necessarily linearly proportional to the energizing the resistors. That is, in a low density region L and a high density region H, the gradient of the curve is small and in contrast that of the curve is large in an intermediate density region which is present between the low and high density regions L and H. This is owing to characteristics of the printing paper such as transfer paper that the sensitivity thereof to heat is small in the low and high density regions L and H.
To obtain desirable printed image of which the density accurately corresponds to the input data, has been developed the conventional apparatus, for example, disclosed in the Japanese Patent Application Provisional Publication No. 61-208366. This apparatus is provided with a signal processing circuit which appropriately selects the increment of the energizing period for energizing the resistors such that the increment of the energizing period corresponding to the change of one level of the density in the low and high density regions is larger than that of the energizing period corresponding to the change of one level of the density in the intermediate density region.
However, the above described conventional apparatus has a drawback that the process of the determination of the increment of the energizing period is complex and thus there is necessity of very troublesome process to determine the optimal conditions in designing the circuits. Further, the conventional apparatus has defects that if the difference between the maximum value and the minimum value of the energizing periods is large, the number of bits required to represent correction data for correcting the increment is extremely increased, that complex configuration of the circuits is necessary and that the processing time of printing the multilevel density image is substantially increased.
Furthermore, the conventional apparatus has encountered problems that as shown in FIG. 21, in case that the density of the printing data is in the low and high density regions in which the sensitivity of the recording paper to heat corresponding to the energizing period is small, it is necessary for obtaining the desirable density of the printed image which further accurately corresponds to the printing data to take longer time to control the energizing period and thereby the printing of the multilevel density image cannot be effected at high speed.
Furthermore, a thermal dye transfer printing apparatus has been conventionally employed as an apparatus for producing a hardcopy of a still picture generated by a duplicator, a facsimile equipment or the like for business or private use. This thermal dye transfer printer employs an inked film formed by applying thermally fusable ink or thermal sublimation ink on a side of a film which is made of polyester and of which the thickness ranges from 5 .mu.m to 6 .mu.m. An inked front surface of this inked film is touched with a recording paper and further a linear thermal head is pressed to the back surface of the paper. When the linear thermal head is energized, it emits heat. Thus, the ink applied at the position on the front surface of the film corresponding to the thermal head is melted and transferred to the recording paper. The thermal head is provided with a plurality of resistors arranged in a line and further the resistors are energized in sequence to emit heat.
The densities representing the gradation of the printed characters, figures and pictures are determined on the basis of areas and densities (that is, number per unit area) of dots formed on the paper by transferring the melted ink thereto. Further, the areas and the densities of the dots of the melted ink are determined on the basis of the quantity of electric current applied to each resistor. In general, the longer a period for energizing the resistor becomes, the larger the quantity of emitted heat becomes, that is, the larger the areas and the densities of the dots become. Thus, as the period for energizing the resistor becomes longer, the density of characters and so forth gets near a saturation value thereof.
FIG. 32 shows an example of the related density controlling apparatus having a data correcting circuit as the above described embodiment of the present invention. In this apparatus, density data are sent from a terminal 327 to a data storage 328. On the other hand, an address counter 329 is supplied with reference clock signals from a terminal 331 and a starting pulse from a terminal 303 and sends addresses (hereunder sometimes referred to simply as first addresses), which are to be fed for the first time, to the data storage 328. Furthermore, the starting pulse is supplied to a mechanism controlling circuit 332 to control an input mechanism. The data storage 328 sends out density data corresponding to the first addresses, that is, density data sent from an analog-to-digital (A/D) converting circuit for the first time to a data correcting circuit 333, which is used to compensate the density data by taking effects of heat generated in the printing operation thereon into account, as described in the Japanese Patent Applications NO. 62-41235 or 62-196855. Further, the data correcting circuit 333 sends the density data corrected by it to a density comparing circuit 334. At that time, the count to be stored in a data counter 335 is set as "1". Then, reference density data, which are sequentially increased in accordance with the count held in the data counter 335, are supplied from a data counter 335 to the density data comparing circuit 334. The density data comparing circuit 334 compares the corrected density data with the reference density data "1" indicating the minimum coloring density. Further, the circuit 334 sends a control data "1" to the linear thermal head portion 336 if the corrected density data is equal to or greater than the reference density data "1" or sends another control data "0" to the portion 336 if less than the reference density data "1".
After the processing of the density data corresponding to the first addresses is completed in the above described manner, the address counter 329 sends second, third, . . . and n-th addresses, which are data to be sent for the second, third, . . . and n-th time, respectively, to the data storage 328 in sequence. The data storage 328 sends out density data corresponding to each of the second, third, . . . and n-th addresses to the density data comparing circuit 334 in sequence each time receives the second, third, . . . or n-th time addresses. The density data corresponding to the first, second, third, . . . and n-th addresses corresponds to printing density data to be printed by the first, second, third, . . . and n-th resistors of the thermal head, respectively. The density data comparing circuit 334 compares the density data corresponding to each of the second, third, . . . and n-th addresses with the reference density data "1" and sends out the control data "1" or "0" to the linear thermal head portion 336 in accordance with the results of the comparison.
When the address counter 329 finishes counting the first, second, third, . . . and n-th addresses, the counter 329 sends a data transferring pulse to both of the data counter 335 and the linear thermal head portion 336. Upon receiving the data transferring pulse, the data counter 335 supplies heating pulses to both the address counter 329 and an AND-circuit 337 at the same time and changes the value of the reference density data from "1" into "2" which is the minimum coloring density data but one. On the other hand, the reference clock signal is supplied from a terminal 331 to a terminal of the AND-circuit 337 which outputs a pulse to the linear thermal head portion 336 simultaneously with the reception of the hearing pulse.
Next, the address counter 329 is reset by the heating pulse and once more counts first, second, . . . and n-th addresses in sequence. Thereafter, n density data are compared with the second reference density data "2" in the density data comparing circuit 334 in sequence. In case that the reference density data is "2", the data counter 355 and the AND-circuit 337 and so on operates similarly as in case that the reference density data is "1" and outputs control data. Further, a heating pulse is applied to the linear thermal head 336 and the resistor corresponding to the outputted density data is energized and emits heat.
However, the above described related apparatus has a defect that if the number of the density levels used to print or reproduce the characters and so on is increased by increasing the number of bits used for representing input density data or if the number of bits required for representing density data to be processed is increased by the data correction circuit 333, the number of bits used for representing the density data to be processed in the data comparing circuit 334 is thus increased and further the configuration of the circuits of the apparatus becomes complex.
The present invention is accomplished to eliminate the above described defects and solve the above described problems of the conventional apparatus or of the related apparatus.
It is therefore an object of the present invention to provide a thermal printing apparatus for printing multilevel density images, characters and figures which can appropriately regulate the levels of the densities used for representing the gradational images, characters and figures printed.
Another object of the present invention is to provide a thermal printing apparatus including a density controlling unit which can obtain sufficient densities of printed images, characters and figures.
A further object of the present invention is to provide a thermal printing apparatus including a density controlling apparatus which can facilitate the correction of the printing data and can increase the quality of the image in the low and high density regions by improving the operation of printing data of which the density is in the low and high density regions.
A still further object of the present invention is to provide a thermal printing apparatus including a density controlling unit which can obtain further greater number of density levels used to print the characters and so on by means of circuits having relatively simple configuration.